1. Field of the Invention
The present invention relates to a surface acoustic wave device using highly oriented diamond films, which is suitable for a high frequency filter.
2. Description of the Related Art
Diamond has a large band gap, and is excellent in stability against chemicals and radiation. It is thus expected to be used for devices operated in harsh environments. In particular, diamond has a property that the acoustic velocity is the highest therein among various materials. Recently, surface acoustic wave devices using the above property of diamond have been proposed in Unexamined Japanese Patent Publication Nos. Hei 1-62911, 1-233819, 1-236712, 1-808948, 2-20910, 2-137413, 2-239715, 3-175811, 3-204211 and 2-299309. Namely, in the above surface acoustic wave devices, by use of a diamond film as a substrate, the propagation velocity of the surface acoustic wave can be higher. Thus it is expected that surface acoustic wave devices in a GHz area can be fabricated.
In the conventional surface acoustic wave devices as a prototype, however, polycrystalline diamond films used as the substrates have a large surface roughness and contain a high density of crystal defects. As a result, the conventional devices fail to demonstrate the high propagation velocity theoretically expected for diamond. Moreover, the rough surface of the polycrystalline diamond film must be polished. As for the crystal defects, an epitaxial film grown on a single crystal diamond seems to contain less defects than the conventional polycrystalline diamond film. However, in this case, there occurs a problem due to its limited size of the substrate, which makes difficult to use the standard photolithography technique, reduce the cost, and achieve mass-production.